1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a powder flow rate, which may be applied, for example, in supplying of a powder coating material.
2. Description of the Related Art
In automatic powder coating, it has been necessary to supply a coating material at a constant rate to achieve a uniform finish of paint coat and to efficiently use the coating material. In order to constantly supply the coating material, various methods for measuring its flow rate have been proposed for use in controlling the powder flow rate. For example, Japanese Patent Publication No.3-7045 and Japanese Patent Laid-Open No.62-64911 respectively disclose methods in which a detecting tube having a uniform diameter is provided in the conveying portion. A predetermined amount of a detecting fluid is blown into an inlet of the detecting tube, and pressures at the inlet and at an outlet thereof are detected to determine the powder flow rate based on difference between the two pressures.
In the method of measuring the powder flow rate by detecting such pressure difference, however, the powder tends to flow, backward into the pressure detecting part especially when, for example, the conveying of the powder is to be stopped. In order to prevent such backward flow of the powder to the pressure detecting part, flushing, for example, is necessary of a purge fluid or the like, requiring a large number of pipes, hoses, etc. Accordingly, the mechanism of the apparatus becomes complicated and the manufacturing cost of the apparatus is thereby increased. A much work time is also required for its installation and maintenance.
Further, while use of a screw feeder or the like is actually practiced to supply the powder coating material at a constant rate, the mechanism for such purpose intrinsically becomes of a large scale. The manufacturing cost of the apparatus is higher. In addition, the portion to be cleaned at the time of changing the color of coating material is complicated in structure, and this is very difficult to be coped with.